1. Field of the Invention
The present invention relates to an antenna system for performing diversity transmission and reception, and relates to a portable terminal having the antenna system.
Priority is claimed on Japanese Patent Application No. 2004-279951, filed Sep. 27, 2004, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIGS. 3A and 3B show a structure of a conventional antenna system for performing diversity transmission and reception, which is mounted in a portable terminal (see Reference 1: Japanese Unexamined Patent Application, First Publication No. 2003-318770).
FIG. 3A shows a state in which a main antenna 2 is contained in a body 1 of a portable terminal, and FIG. 3B shows a state in which the main antenna 2 is pulled out of the body 1 of the portable terminal. In FIGS. 3A and 3B, the antenna system has the main antenna 2 and a sub-antenna 3, so as to perform diversity reception.
The main antenna 2 includes a helical antenna 20 and a linear antenna 23, which constitute a whip antenna. The helical antenna 20 is provided at the head of the main antenna 2 and is surrounded by an insulating material 21. An end of a coil of which the helical antenna 20 is made is connected to a terminal 22.
The linear antenna 23 is surrounded by an insulating material 24, and an end of the linear antenna 23 is connected to a terminal 25.
The body 1 of the portable terminal contains a power supply 100 for supplying electric power to the main antenna 2, a bandpass filter 101 for passing a transmitting or receiving signal of a desired frequency, a switch 102, the above sub-antenna 3, and a bandpass filter 103 for passing a transmitting or receiving signal of a desired frequency.
The power supply 100 is connected via the filter 101 to the switch 102. According to the movement of the main antenna 2, the power supply 100 can be positioned where the power supply 100 contacts the terminal 22 or 25.
The sub-antenna 3, which is a dipole antenna, has a coil wound around an insulating body, and electric power is supplied to a center point of the coil. The center point for supplying electric power is connected via the filter 103 to the switch 102. In contrast with the main antenna 2, the sub-antenna 3 is fixed in the body 1 so that the constitution of the sub-antenna is not changed even when the main antenna 2 is pulled out or contained.
In the above structure, when the main antenna 2 is pushed into the body 1 of the portable terminal and is contained in the body 1 (see FIG. 3A), the terminal 22 of the main antenna 2 contacts the power supply 100, so that the power supply 100 and an end of the helical antenna 20 are connected to each other.
Accordingly, only the portion indicated by the double-headed arrow “a”, that is, only the helical antenna 20 functions as the main antenna. FIG. 4A shows a directional pattern of the antenna in this state. In FIG. 4A, the ordinary solid line shows a directional pattern of the main antenna, and the bold solid line shows a directional pattern of the sub-antenna. As clearly shown in FIG. 4A, the area having low directivity in the main antenna is covered or compensated for by the directivity of the sub-antenna.
In contrast, when the main antenna 2 is pulled out from the body 1 of the portable terminal (see FIG. 3B), the terminal 25 of the main antenna 2 contacts the power supply 100, so that the power supply 100 and an end of the linear antenna 23 are connected to each other.
Accordingly, only the portion indicated by the double-headed arrow “b”, that is, only the linear antenna 23 functions as the main antenna. FIG. 4B shows a directional pattern of the antenna in this state. In FIG. 4B, the ordinary solid line shows a directional pattern of the main antenna, and the bold solid line shows a directional pattern of the sub-antenna. In this case, as the main antenna is switched from the helical antenna 20 to the linear antenna 23, directivity of the main antenna is improved, and an improved directional pattern of the main antenna substantially covers the directional pattern of the sub-antenna.
In the above antenna system as described in Reference 1, the sub-antenna is fixed in the body of the portable terminal. Therefore, when the main antenna is pulled out or contained, that is, when the main antenna is pulled out from the body of the portable terminal or contained in the body, the directional pattern of the main antenna changes while the directional pattern of the sub-antenna cannot be changed.